To follow the processes of drug delivery and degradation of the delivery device in vivo noninvasively by EPR and MRI. The aim of the study is to characterize the mechanisms underlying drug delivery (bulk hydrolysis vs. surface erosion) and to find out the reasons for the differences of in vivo/in vitro studies. The current studies cover a broad range of biodegradable polymers used for dr ug delivery devices which are on the market (polylactide-co-glycolide) or are FDA approved for studies on humans (polyanhydrides such as PCCPP-SA) or P(FAD-SA). By means of EPR it is possible to obtain information concerning microviscosity , micropolarity and pH value of the implant in vivo. MRI is used to charact erize the water penetration and changes in the form of the tablet.